It's all about cooling. If you can keep it cool, and running stable while mining it won't hurt your GPU's any more than intense gaming would. Long gone are the days where a GPU fan is a 5 cent piece of junk prone to failure in 6 months. Most of them are fairly heavy duty now. If you mine 24/7 be sure that once a month you clean all the fans / filters in the case especially if you have pets. Clogged fans will do more to blow up your PC than anything else. That goes for rigs with or without a case. You'd be surprised how many dead bugs came out of my CPU heat sink after 1 month of open box mining.